First of all it's possible that SPD can develop later in life. For a personality to shift from one to another and become a disorder it requires certain events to happen in someones life. SPD is not a brain disorder. It's a personality disorder. And personality is mostly dictated by parenting style, environmental factors, life events, and lastly brain trauma/disease.
I rarely think of what I wrote here. On certain rare occasions these memories and emotions rise to the surface. In this case it was when I stopped taking medication. I think alot of schizoids have no idea why they are schizoid because the very nature of the disorder makes it difficult to become aware of ones one emotions and memories. I just wrote this today, because I saw it as an opportunity to study myself.